1. Field of the Invention
Aspects of the invention relate to providing increased protection for personnel working on low voltage electric power circuits, and more particularly, to providing a maintenance trip function that overrides the normal trip function to reduce arc energy in faults occurring during maintenance.
2. Background Information
Often, maintenance personnel need to work on low voltage electric power circuits that are energized, for instance, when testing circuit breakers. The circuit breakers have trip functions designed to protect the power system from overloads and overcurrents. These trip functions typically include a delayed trip function that allows downstream breakers closer to the fault to respond first in order to limit interruption of service. The delayed trip function also permits temporary overloads such as those associated with the starting current of large motors.
The coordination between circuit breakers can be enhanced by zone interlocking in which a breaker lower in the protection hierarchy sends a signal that it detects a fault to an upstream breaker to temporarily block operation of the latter breaker giving the downstream breaker time to respond. Absence of a zone interlock signal from the lower breaker indicates that the fault is above the lower breaker allowing the higher breaker to respond more quickly.
Often, the power breaker will also have an instantaneous trip function that responds without delay to very high currents, such as those associated with the short circuit. These breakers can also have a ground fault trip function that typically also incorporates a delay time.
Maintenance and test personnel can be exposed to live terminals when working or making measurements on energized gear. The potential for arc flash resulting from an accidental fault requires that personnel wear protective clothing while working on energized gear.